Discussion 4/10
AOSC200
Sarah Benish
Outline
 I. Air Masses
 II. Fronts
 III. Mid-latitude Cyclonic Storms
I. Air Masses
 What is it?
 Extremely large body of air with similar temperature and
humidity in any horizontal direction at a given altitude
 What are their source regions?
 Where air masses originates; air mass will acquire
properties of the source below
 Either continental (c) or Maritime (m)
 How do we classify them?
 Usually in terms of temperature and humidity and source
region
cP and cA Air Masses
 Cold and dry
 Where does cP air come from?
 Canada, can head as far south as FL
 Where does cA air come from?
 Forms over frozen Arctic bringing bitterly cold
temperatures
 Sometimes called the “Siberian Express”
 Strong winds and blowing snow can lead to blizzards
mP Air Mass
 Where do they form?
 Over oceans at high latitudes, cool to cold and humid
 In winter, mP from Pacific begin as cP from Siberia
 These systems run into west coast mountains and
dump lots of snow
 What lifting mechanism is this called?
 On East coast, mP brings in moist air from Atlantic over
land where is meets with cP air
 What does this lead to?
 SNOW!
mT Air Mass
 Where does it form?
 Forms over Gulf of Mexico, Caribbean Sea, and
subtropical western Atlantic Ocean
 Eastern U.S. strongly affected
 Stable air mass leads to oppressive heat wave
 Wintertime precipitation over central and eastern U.S.
due to uplift of mT over cold air masses
 Western U.S. strongly affected by mT air from Pacific
cT Air Mass
 Where does it form?
 Form over tropical and subtropical deserts and plateaus
 Air mass is hot and dry
 When cT and mT air meet, contrast between systems
is called a “dry line.”
 In summer, large supercell storms often form at the dry
line conducive to tornado development
Air Mass Modifications
 How can air masses be modified?
 Lifting
 Heat exchange with the surface
 Example: Lake Effect Snow
Lake Effect Snow
II. Fronts-Cold
Cold Fronts cont.
Warm Fronts
Warm Fronts cont.
Stationary Fronts
 Essentially no movement
 What does it look like?
 Winds blow along the front but in opposite directions
 Usually no precipitation (air masses are dry)
Occluded Fronts
 What is it?
 Cold front catches up to and overtakes warm front
 Warm air is forced up over both cold/very cold air
masses
 May have a mix of clouds similar to both warm and cold
fronts
Occluded Fronts (cold type)
cP air overtaking mP air
Occluded Fronts (warm)
mP cool air replacing cold cP air
Occluded cont.
What are cold, warm, and occluded fronts often a part of? Mid-latitude cyclone
III. Mid-Latitude Cyclonic
Storms
 Cyclogenesis—development or strengthening of a midlatitude cyclone
 Frontal wave—wavelike deformation along a front in
the lower levels of the atmosphere. Those that develop
into storms are stable waves; those that don’t are
unstable waves.
 Open wave—stage of development of a wave cyclone
where a cold and warm front exist, but no occluded
front. The center of lowest pressure in the wave is
located at the junction of the two fronts.
Cyclone Model (Bjerknes)
Birth: begins as a frontal wave along stationary front
separating cP air from mT air
Young adult: Open wave develops strong cold and warm
fronts, precipitation falls along broad area
Mature (occluded cyclone): occluded front develops,
pressure reaches minimum, winds reach maximum
Death: cut-off cyclone develops, pressure rises, clouds and
precipitation dissipates
Key Terms
 Air mass
 Lake-effect snow
 cT, mT, cA, mA, cP, mP
 Front
 Cyclogenesis
 Frontal wave
 Open wave